Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current.
The CdS layer, along with other layers (e.g., a transparent conductive oxide layer of cadmium tin oxide) can be formed via a sputtering process (also know as physical vapor deposition) where the source material is supplied from a semiconducting target (e.g., cadmium sulfide, cadmium tin oxide, etc.). Typically, the cadmium sulfide semiconducting target is bonded to a backing plate that is water cooled and then placed into magnetrons (cathodes) that perform the sputtering action. The semiconducting target is typically bonded to the backing plate using indium solder or a conductive epoxy. The bond provides good thermal and electrical contact between the semiconducting target and the water cooled backing plate. Thus, the heat created by the plasma on the opposite side of the semiconducting target can be dissipated and carried away from the target by the water cooled backing plate.
As the semiconducting target is sputtered, the semiconducting material is eroded from the target. As the semiconducting target erodes, nodules form on the surface of the target that, over time, may change the deposition rate during sputtering and could affect the characteristics of the resulting thin film. Additionally, these nodules can cause arcs to form in the sputtering chamber. These variables created after sputtering over an extended period can lead to thin film variances of the deposited semiconducting layers in a large-scale, manufacturing environment, such as during the commercial manufacture of cadmium telluride based thin film photovoltaic devices.
Thus, a need exists for a more uniform sputtering process for the deposition of substantially uniform layers.